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Improving process stability and kinetics of anaerobic biowaste digestion by promoting direct interspecies electron transfer (DIET) among syntrophic microbial consortia
Microbial conversion of organic matter to renewable energy in form of methane is a proven and widespread strategy for effective waste management. In such methane-producing environments, electrical connected bacteria and archaea perform direct interspecies electron transfer (DIET) as alternative syntrophic mechanism to interspecies hydrogen or formate transfer. However, fundamental aspects of the microbial ecology concerning DIET and its significance for biogas production are still unclear. The goal of this research was to elucidate the role of direct interspecies electron transfer among syntrophic short-chain fatty acid-oxidizing consortia for the methanation of organic matter. Within this project we contributed to a more widely applicable knowledge about structure-function relationships of syntrophic core communities in mesophilic and thermophilic digesters by integrating molecular tools such as the 16S rRNA approach, meta-genomics and -transcriptomics with cultivation-based techniques. A genome-centric metatranscriptomic approach refutes a major role of DIET in the studied systems and rather suggests electron transfer via formate and hydrogen. However, we achieved fundamental new insights into the ecophysiology of syntrophic propionate- and acetate-oxidizing bacteria in bioreactors. Due to unfavourable thermodynamics, the fermentation of propionate and acetate can be considered as bottleneck in anaerobic digestion of organic matter. Metagenome sequencing and enrichment cultivation revealed novel players in the syntrophic turnover of those quantitatively important intermediates. Strikingly, members of the poorly understood candidate phylumCloacimonetes were identified as integral part of the syntrophic community in mesophilic anaerobic digesters. In thermophilic dry fermentation of biowaste diverse and novel syntrophic acetate-oxidizing bacteria are important for process functioning and overall organic carbon mineralization.